Loss of sensitivity to negative growth regulators may be an important step in the development of malignant tumours. Transforming growth factor .beta., (TGF.beta.), a potent natural antiproliferative agent, is believed to play an important role in suppressing tumorigenicity. Comparisons of human colon carcinoma and melanoma cell lines have demonstrated a progressive loss of responsiveness to the growth inhibitory effects of TGF.beta. as tumour aggressiveness increases (Filmus et. al., 1993; Roberts et. al., 1993). Further, in certain tumour cells that have escaped TGF.beta. regulation, tumour aggressiveness has been directly correlated with an increased ability to secrete TGF.beta., which may act in a paracrine manner to promote angiogenesis and inhibit the immune response (Roberts et al., 1993). Thus, an understanding of the molecular events associated with loss of TGF.beta.-responsiveness in tumours could provide major insights into the general mechanisms underlying the development of malignancies. At present, the mechanism for the escape from TGF.beta. regulation is not clear; mutational inactivation of components of TGFP signalling pathways could be one mechanism underlying acquisition of TGF.beta. resistance.
TGF.beta. signals through heteromeric receptor complexes of type I (T.beta.RI) and type II (T.beta.RII) serine/threonine (Ser/Thr) kinase receptors (Massague et al., 1994; Miyazono et al., 1994). Receptor activation occurs on binding of ligand to T.beta.RII which then recruits and phosphorylates T.beta.RI which propagates the signal to downstream targets (Attisano et al., 1996; Chen et al., 1995; Wrana et al., 1994). Several studies have indicated that alterations in receptor expression or function may be involved in some cancers. For example, in a subset of colon cancer cell lines that display high rates of microsatellite instability, and in several TGF.beta.-resistant human gastric cancers, genetic changes in the type II receptor have been identified (Markowitz et al., 1995; Park et al., 1994). However, since the intracellular targets of the TGF.beta. receptors are poorly understood, the precise mechanism by which the disruption of TGF.beta. signalling pathways results in promoting tumorigenesis is unclear.
Recently, MADs (Mothers against dpp, decapentaplegic gene) and MADR (MAD-related proteins) have been identified in a variety of species as important components of the signal transduction pathway required for serine/threonine kinase receptor signalling (Graff et al., 1996; Goodless et al., 1996; Newfeld et al., 1996; Savage et al., 1996; Sekelsky et al., 1995; Thomsen, 1996). The Drosophila protein, MAD, and the closely related vertebrate protein MADR1, appear to be essential for signalling of the decapentaplegic (DPP) and bone morphogenetic protein 2 (BMP2) pathways, respectively, and MADR1 can specify BMP (bone morphogenetic proteins) specific biological responses (Graff, et al., 1996; Hoodless et al., 1996; Newfeld et al., 1996).
MADR1 is rapidly and specifically phosphorylated by the BMP2 pathway and not by the TGF.beta.-induced pathway (Hoodless et al., 1996). Furthermore, MADR1 redistributes from the cytoplasm to the nucleus upon introduction of signalling suggesting that MADs may have a nuclear function (Hoodless et al., 1996). Recently, a search for tumour suppressor genes implicated in pancreatic cancer led to the identification of the MAD-related gene, DPC4 (deleted in pancreatic carcinoma, Hahn et al., 1996a). However, the signalling pathway in which DPC4 functions is unknown.
A novel MAD-related gene, MADR2, has now been identified, sequenced and characterized as a candidate tumour suppressor gene. MADR2 protein has been shown to be regulated by TGF.beta.. Mutations of the MADR2 gene have been associated with colorectal carcinomas, strongly implicating the MADR2 protein in the development of human tumours. Knowledge of the function and localization of the mutated protein is useful in devising therapeutic strategies that are highly specific for neoplastic cells.